to get it, you just have to keep watching it and playing it. ive read every aspect of the rule book (nfl although college and other levels are different) and its not a fun read. and then tactics and strategy isnt something you can really explain either. it takes about a good year or 2 to grasp how to play and then it takes a few more to actually learn a lot more about the sport and how it works. i dont want to be a sell out and give you a generic answer, but thats really the best way to learn. in most other sports, i can explain the rules and how they work. but since i feel bad, heres a really general overview of the nfl:
11 players on 2 sides. on offense, you must have at least 7 players on the line of scrimmage and 5 of them must be linemen. then there a multitude of positions (skilled) that can go on or off the line. on a base offense, it consists of 2 tackles, setting the outside of the line, 2 guards on the inside and the center who snaps the ball and the center of the linemen. ad then theres a 6th linemen/skilled player called a tight end. and then you have the qb, a full back, a running back and finally a wide receiver. now, there a multitude of formations with more or even none of those skilled positions. a play is broken down into 2 categories: run and pass. but there are different ways to set that up. for instance, you have under center (qb put his hands under the centers but to get the ball) or a shot gun formation (the qb is 3 or 4 yards behind the center. and out of these 2 kinds of plays, you do a combination. a running play made to look like a passing play is called a draw play while a passing play made to look like a running play is called play action. the purpose of the 4 can be a loaded thing to explain so i'll stop with that. then you have 4 chances to move the ball 10 yards (a down for distance system). once you get 10 yards, it resets and you have 4 more tries to get those 10 yards. there is a lot more to explain, but thats in its most simplistic form
the defense is slightly more complicated. you have different kinds of defense. a base defense is a 43 defense (4-3: 4 linemen and 3 linebackers). in that, you have nose tackle, defensive end, defensive tackle and and another nose. the linebackers are the weakside, middle and strongside (if you will, will, mike and sam respectively or outside and inside linebackers). and then you have your defensive backs who are basically the last resort of the defense. you have 2 safeties: strong and free safety (i'll get into the difference of those 2 later) and 2 corner backs. linemen rush and try to sack the qb or stop the running back in the backfield or for minimal gain on a down. a linebacker as you can figure backs the o line, but can also cover to prevent passes. corners cover receivers like i said and safeties are the safety net of the defense. a strong safety is more like a linebacker and usually hits harder while a free safety is more of a corner and basically stays in the backfield. now, you can change the number of linemen, linebackers and defensive backs, but i dont feel like really explaining it because it takes a while. in coverage, you have either man or zone coverage. many times, its a combination of both. you have corners playing man with the wide receivers and the safeties hanging back, looking at the play develop, playing zone. just for a little pointer, cover # means how many people are playing zone. cover 1 means everyone whos covering is playing man coverage except 1 person (usually a safety) playing zone.
and then you have special teams and all that and you can look that up yourself. other generic information is the point system:
field goal: 3 points
touch down: 6 points
extra point: 1 point
2 point conversion: 2 points
however, missing any of them means no points for the team (im mainly talking about the extra point system).
usually, you have teams doing 3 plays on the 1st 3 downs and then on 4th down, unless its necessary (usually on 4th in short or inside the last 2 minutes of the half), teams will punt or go for a field goal.